Characters added in the middle of a TV show’s run get a bad rap. For every Cousin Oliver or Scrappy-Doo, there are characters who actually added something to their show, even rejuvenating a longrunning series’ later seasons. Some start as stand-ins for departed cast members, then take on expanded roles—others begin as guest stars in one season, before returning as regular players down the line. (In rare instances, sometimes they’re the sole reason for watching the declining years of a formerly great show.) But one thing unites these anti-Cousin Olivers and non-Scrappy-Doos: They helped keep their shows afloat—some to the very end.

In 1987, the producers of Cheers faced an impossible dilemma. The show had become one of the most popular sitcoms on TV, in large part because audiences were so invested in the show’s central romance. Then Shelley Long, the show’s female lead, left the show for a movie career that never completely panned out. The production team took a risk in replacing Long with a relative unknown: Kirstie Alley, who had only done intermittent television work since her screen debut in Star Trek II, little of it comedy. But rather than try and recreate Long’s character and her chemistry with Ted Danson, Cheers wisely pivoted into a different kind of show. What had been a screwball romantic comedy was reinvented as a workplace sitcom, with Alley’s steely corporate striver Rebecca Howe in the villain role (until the cracks in her professional facade grew wider and wider over time). The rejuvenated series ran for six years with Alley in the lead—a year longer than Long’s tenure on the show. [Mike Vago]

The last great NBC Thursday sitcom managed to survive for seven years despite low ratings, in no small part because of frequent reinventions. After a brief, poorly received first season, the character dynamics and tone were tweaked for a much-improved second. But the show didn’t really hit its stride until the third season. The series’ producers needed to replace a departing Paul Schneider, whose low-key demeanor no longer fit the show’s energetic tone. They supposedly reached out to both Rob Lowe and Adam Scott, not sure they could get either actor, and ended up with both. Lowe’s unflappably cheerful city manager Chris Traeger was an inspired doubling down on Amy Poehler’s overenthusiastic lead Leslie Knope (and Lowe gracefully bowed out after three years, once the writers started to run out of material for the character). But it was Scott’s tightly wound but thoroughly decent government auditor Ben Wyatt who became central to the show and its creative success, as he became both a well-matched love interest to Leslie, and an ideal straight man to the rest of the cast. [Mike Vago]

The addition of Lane Pryce to the world of Mad Men saved it from a purely practical standpoint. Without his collusion in creating Sterling Cooper Draper Pryce, after all, the ad agency would have been slashed and sold off, its central players scattered to the wind. Lane was also instrumental to keeping the whole ship afloat while everyone else got sozzled and screwed around; as Don Draper, Roger Sterling, and Bert Cooper all admitted, they had no idea “how to do what he does.” But more than just being the guy who kept the lights on, Lane gave the AMC drama new life, becoming one of Mad Men’s most fascinating—and, ultimately, tragic—figures. Jared Harris imbued Lane’s every stiff Britishism with humor and heartbreaking pathos, as his polite desperation to connect (with his colleagues, with his “chocolate bunny,” with Joan, with America) was repeatedly stifled and, finally, strangled out of him. And Lane’s story arc provided many of the series’ most memorable moments, whether it was getting drunk at Gamera with Don, punching Pete in the face, or his heart-rending exit. Though the partners did their best to scrape his name off the door and forget him in its final seasons, the loss of Lane haunted both the agency and the show long after he was gone. [Sean O’Neal]

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4. Valerie Malone, Beverly Hills, 90210

When Shannen Doherty exited Beverly Hills, 90210 in 1994, the show had some big—and bitchy—shoes to fill. Mission accomplished: It compensated for her absence (and then some) by adding Tiffani Thiessen, who obliterated the innocent vibes of her Saved By The Bell character Kelly Kapowski by playing the devious, perpetually scheming Valerie Malone. Although Valerie’s storylines often resembled outlandish soap-opera moments—e.g., being arrested for impersonating a prostitute, heading to Mexico to recover Dylan’s stolen money and little sister, sabotaging relationships left and right—they added romantic sizzle, creative friction, and interpersonal chaos that kept the show interesting and overflowing with drama. By being a not-so-innocent foil, she also helped 90210 move into a darker, more grown-up direction—appropriate given that during the Valerie era, the entire gang was either attending college or navigating adulthood. [Annie Zaleski]

It’s common knowledge that Joss Whedon’s greatest strength is his ensemble writing, which makes the first season of Angel, which boasts a measly three regular cast members, surprisingly dull at times. That cast filled out in the second season, and the show improved, but it was only at the very end of that second season—when Amy Acker joined the Angel Investigations team as Winifred “Fred” Burkle—that Angel really became Angel. Some of that comes from the ensemble she made whole, some of it from the Angel writers taking the serialization brakes off in seasons three and four. But a tremendous amount of credit goes to Acker, who consistently gives her scenes warmth and focus, whether playing the unhinged naif or the competent researcher—or far darker personas when that becomes necessary later in the run. [Rowan Kaiser]

Replacing an unqualified Kevin Malone (who in turn relieved an uninterested Pam Beesly), Erin Hannon filled an entirely new position at Dunder Mifflin Scranton: A receptionist who liked her job. In the twilight years of The Office, Erin was an all-too-rare bright spot, the energy of Ellie Kemper’s performance often enlivening a series that became freighted with secondary characters and frantic storytelling shortly after her arrival. She wasn’t immune to these qualities, but Erin’s innate sweetness and Kemper’s canniness with a punchline (see: the “disposable camera” gag from season seven’s “Counseling”) kept the character from getting completely sucked into vortexes of awfulness like the reign of James Spader’s Robert California and the devolution of one-time beau Andy Bernard. If only the same could be said of the times Erin wasn’t on-screen. [Erik Adams]

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7. Sue Ann Nivens, The Mary Tyler Moore Show

Boasting one of the finest TV ensembles ever assembled, The Mary Tyler Moore Show could’ve been a victim of its own success, losing two key players (first Valerie Harper’s Rhoda, then Cloris Leachman’s Phyllis) to separate spin-offs in successive seasons. Fortunately, the producers brought backup: The fourth-season premiere, “The Lars Affair,” introduced Betty White as Sue Ann Nivens, chipper host of WJM’s Happy Homemaker (and, in her off-hours, an unrepentant homewrecker). Ed Weinberger’s script described Sue Ann as “an icky sweet Betty White type,” a fit for the actor who rose to prominence hosting NBC’s presentation of the Tournament Of Roses Parade—but the character came to define a completely new “Betty White type.” Sue Ann’s TV persona hid a mean streak and a healthy sexual appetite, a one-woman Jekyll-Hyde act expanding the horizons of Mary Tyler Moore’s workplace and setting up the second act of White’s career. She could gab about guys like Rhoda did and hand out a backhanded compliment like Phyllis did, all in that signature, “icky sweet” singsong that gained a shot of venom thanks to Sue Ann. [Erik Adams]

Chevy Chase’s departure from Community at the end of its fourth season was essentially planned turnover. Chase’s bitter feud with creator Dan Harmon was well-documented, and Pierce Hawthorne was the character most easily carved out of the Greendale gang without impacting the show’s rhythm. But Donald Glover’s departure less than half way through the fifth season was a serious blow, and when Yvette Nicole Brown dropped out of Greendale at the end of the season, the community was looking more like a cadre. But Harmon bowled a turkey with his late additions: Professor Buzz Hickey (Jonathan Banks) who joined in season five, and Frankie Dart (Paget Brewster) and Elroy Patashnik (Keith David), who joined in season six when Banks left the show along with Brown. Each character compensated for what the departed characters took with them, but in a subtle way that never made them seem like imitations. It helps that each character was brought to life by actors with crack comic timing, which is even funnier since none of them looks the part. [Joshua Alston]

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11. Pearl Forrester, Mystery Science Theater 3000

Mary Jo Pehl joined the staff at Best Brains Inc. for Mystery Science Theater 3000’s fourth season, contributing riffs in the writers’ room and narrating the action aboard the Satellite Of Love as the disembodied Magic Voice. She appeared in a number on-screen roles (like the similarly disembodied Jan In The Pan), too, eventually succeeding villainous sidekick TV’s Frank in the role of Pearl Forrester. The mother of MST3K villain Dr. Clayton Forrester and assistant in his deranged quest for world-domination-through-cinematic-garbage, Pearl was a minor cog of contingency in the show’s abbreviated seventh season, but she jumped several rungs up the global-conquest ladder during MST3K’s transition from Comedy Central to Syfy (then the Sci-Fi Channel). Snuffing Clay out between seasons, Pearl didn’t merely inherit her son’s mad-scientist mantle—she kept the show’s entire premise intact, force-feeding crappy movies to Mike Nelson and his robot friends for three more seasons. Despite being Nelson’s captor, Pearl developed a playful antagonism with the SOL crew, eventually taking her own seat in the theater for the first half-hour of the medieval stinker Quest Of The Delta Knights. [Erik Adams]

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12. Seven Of Nine, Star Trek: Voyager

Over the years, Star Trek has generated some of its best stories and characters by contrasting hyper-competent, analytical outsiders against casts full of well-meaning, humanity-cheering do-gooders. Of the structural sins committed by the franchise’s bland fourth outing, the lack of one of these Spock-type character might be the worst. Aware of this weakness, the show’s producers started Voyager’s fourth season by knocking off their most superfluous character—alien pixie woman Kes—and replacing her with one designed to merge the old outsider archetype with some good old-fashioned sex appeal: former Borg drone Seven Of Nine. As a victim of decades of technological brainwashing, Seven (played with glacially thawing coldness by actress Jeri Ryan) spent her time on the ship poking at her crew member’s moral certainties, sneering at the weakness of individuals, and sparring over ethics with Captain Janeway. Her slow recovery from the Collective also gave Seven an actual character arc, a rarity on a show full of genial, static characters, and helped her to become the show’s most reliable narrative engine, as well as its most recognizable face. It didn’t hurt that the producers stuck former Miss Illinois Ryan in a form-fitting catsuit for the part, then plastered images of her all over magazine covers in an attempt to lure viewers back to the show. [William Hughes]

The fourth season of the third live-action Star Trek series saw a host of changes intended to enliven what was already a thoughtful, character-driven show: Avery Brooks got to finally shave his head and grow a suitably badass beard as the newly promoted Captain Sisko, the theme song got an energetic redux, and the Dominion War threatened to go from cold to hot at any moment. But the biggest change of all was the addition of Michael Dorn’s Worf, who joined the show following the end of Star Trek:The Next Generation. Of all the Star Trek spin-offs, Deep Space Nine started the strongest and maintained the highest quality over the course of its entire run, so there’s an argument that the show didn’t needWorf in the same way that Voyager desperately needed the jolt Seven Of Nine provided. But Worf provided the show with yet another character capable of carrying entire episodes, adding still more to its storytelling arsenal. Besides, it’s damn hard to imagine the Dominion War storyline reaching its greatest heights without the franchise’s most fearsome warrior along for the ride. [Alasdair Wilkins]

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14. Alan Shore, The Practice

The Practice ran for eight seasons on ABC, but its ratings began to nose-dive in its sixth year. The declining interest forced budget cuts, and several series regulars—including Dylan McDermott and Lara Flynn Boyle—were fired before the start of the eighth season. So there was a lot riding on James Spader when he joined the show in 2003 as Alan Shore, a soft-spoken attorney who regularly used extralegal measures in defense of his clients. It turns out he was more than up to the task: He quickly established Shore as someone who was unaccountable and impossible to keep up with—in the courtroom and in regular conversation. This rubbed his on-screen co-workers the wrong way, and Shore was subsequently fired. But Spader had righted the ship enough for David E. Kelley to send him on to his own show, Boston Legal. Spader won three Emmys while on the spin-off, which featured Shore at a new firm with his best buddy Denny (William Shatner). [Danette Chavez]

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15. Simka Dahblitz-Gravas, Taxi

Andy Kaufman was largely uninterested in the act of making Taxi, especially as the show went on. In order to keep his character Latka Gravas an active part of the show, the producers brought in a love interest in the form of Simka (Carol Kane), a girl from his home country. In addition to keeping pace with Kaufman’s nonsense speak, Simka reinforced Latka’s humanity in her early appearances in season two’s “Guess Who’s Coming For Brefnish” and season four’s “Simka Returns.” Added as a regular in season five after she married Latka, Simka helped anchor the show, driving stories about infidelity and immigration despite her heavily comedic speech patterns. [Les Chappell]

As in many other teen dramas, the transition from high school to college was rough for Dawson’s Creek. But luckily, there was a shining light from the moment the Capeside Five headed for the big cities: Audrey Liddell (Busy Philipps). Introduced as a recurring guest star and Joey Potter’s (Katie Holmes) sexually liberated, rich-girl roommate at Worthington College, Audrey’s carpe-diem attitude initially frustrated neurotic Joey. But she quickly won the hearts of the other series regulars—she even dated Pacey (Joshua Jackson)—as well as the audience: She was fun, and the Capeside crew had long ago forgotten what “fun” even was. Also, Audrey was the only character who wasn’t afraid to be honest with Joey when it came to her Dawson nonsense, which reached an all-time melodramatic high during the show’s freshman year at Worthington. In a fifth season full of Chad Michael Murray, killer ice cream cones, and Joey Potter “singing,” Audrey was the MVP. Of course, season six reminded us that all good things must come to an end (as the title of the series finale goes). That final season decided there needed to be consequences for Audrey’s happy-go-lucky lifestyle: She turned to drugs, alcohol, and Jack Osbourne in a fit of depression, becoming as messed up as every other character on Dawson’s Creek. [LaToya Ferguson]

17. Taylor Townsend, The O.C.

Though Mischa Barton’s Marissa cast a heavy, brooding shadow over The O.C., her season-three death actually led the show to one of its most refreshing revamps. Previously introduced as just another neurotic Tracy Flick-style Newport resident, Autumn Reeser’s Taylor Townsend was given a bigger role for the show’s fourth season, ultimately becoming the romantic yin to outsider Ryan Atwood’s yang. With her effusive personality and eccentric quirks, including a much-older and very French ex-husband, Taylor Townsend made The O.C. fun again for its last season, something the show so desperately needed. [Marah Eakin]

Characters leaving high school was an especially daunting prospect for a show like Friday Night Lights, which spent so much time making its audience fall just as much in love with the kids as it did the adults. But Smash Williams couldn’t hang around Dillon forever. Perhaps the show’s smartest move was switching Coach Taylor over to another school, East Dillon, where both he and the audience could start fresh with a new team. It allowed Friday Night Lights to retain some fans favorites (Landry!), while welcoming new faces (Michael B. Jordan’s Vince, Matt Lauria’s Luke Cafferty) whose eyes were just as clear, and hearts were just as full as their predecessors. [Molly Eichel]